Lithium-ion batteries are capable of providing high charge densities and a high specific charge.
Undesirable chemical changes on the electrode material, which are induced by the charging/discharging operation, are the cause of the inadequate cycle stability of batteries. This is accompanied by a loss of electrical power. For example, in the case of lithium-ion batteries, e.g., NCM (nickel-cobalt-manganese oxide) or HE (high-energy) NCM battery types, due to the intercalation process of the lithium in addition to solvent molecules, substantial volume changes of the battery active material and possibly elutions of individual metals may occur. In particular lithium-sulfide types (Li/S types) have mechanical cracks in the active material or losses/changes of the electrode surface in general as a result of the charging/discharging operations. For Li/S types, diffusion of the polysulfides out of the cathode may occur, which may then react with the lithium metal electrode in the most unfavorable case. All of these undesirable secondary reactions share the feature that the electrode itself and in particular the surface structure of the electrode is modified in such a way that substance losses or structure changes may be induced, which significantly reduce the performance capacity of the battery and in the worst case result in destruction of the entire cell structure.
A feasible path for mechanical stabilization of the electrode and in particular of the electrode surface is the addition of polymers to the active material. The polymers may be mixed with the electrode active material and thus stabilize the mechanical cohesion of the active material. They “bind” the active material and are therefore also referred to as electrode binders. Electrode binders in general ensure the mechanical stability of the electrodes and the contacting of the particles made of electrochemically active material with one another and with the current arrester. Furthermore, conductivity-improving additives may contribute to improved electrical connection of the electrochemically active particles to the current arrester. All additives should be electrochemically stable at least in the potential range of the particular electrode and should have a chemically inert character in relation to common electrolyte solutions.
Furthermore, polymers may also accumulate solidly on electrode surfaces and form a protective layer thereon, similar to the so-called SEI (solid electrolyte phase) on anodes. This intermediate layer coats the entire surface of the active material and prevents an excessively strong volume change as a result of the running intercalation operations. In the most favorable case, this protective layer has the result that only the small lithium ions may still diffuse into the active material and solvent molecules or other decomposition products may no longer interact with the active material. This external layer is thus similar in its function to a solid electrolyte which is permeable to lithium ions.
Polyvinylidene fluoride (PVDF) is added as a standard electrode binder in lithium-ion batteries. The PVDF stabilizes the electrode in a purely mechanical way; however, the formation of an electrode-protecting SEI may not be achieved via the addition of PVDF. In addition, it is disadvantageous that the mechanical stabilization is inadequate and only unsatisfactory cycle stability is achieved in the application.
One way of improving the situation is described, for example, by DE 1020 0800 0113. In this patent application, anode and/or cathode compounds for use in a lithium-ion polymer cell and the use thereof for polymer cells are described, which are characterized in that the electrode compounds contain a binder based on polyalkali silicates and are coated as dispersions on primed electrode arresters.
Furthermore, DE 10 2011 113166 provides a method for producing a cathode of a liquid electrolyte capacitor, the method including the application of a precursor solution to a roughened surface of a metal substrate and the precursor solution containing a substituted thiophene monomer and an oxidative catalyst. The oxidative catalyst is used in a less-than-stoichiometric quantity and the polymerization of the substituted thiophene monomer is carried out with formation of a coating which contains an intrinsically conductive substituted polythiophene.
Patent document US 2010 0143767 A1 discusses, in contrast, an electrolyte composition made of a solvent, a strongly branched polymer, and a polymer having high ionic conductivity. The polymer having high ionic conductivity and the strongly branched polymer are uniformly distributed via the solvent and the strongly branched polymer has a branching factor of greater than 0.5.
In the more recent scientific literature, for example, Kovalenko, I. et al. (Science Vol. 334, 2011, pages 75-79), approaches are additionally found in which brown algae extracts are provided for the exclusive construction of anodes in lithium-ion batteries. It is described in this citation, inter alia, that a lithium nanopowder having alginates, i.e., natural polysaccharides extracted from brown algae, may be used for constructing stable battery anodes, the capacitance of which is approximately eight times higher than that of presently known anodes based on graphite.